Various industrial robots including articulated industrial robots each having articulated functional units, such as a plurality of robot arms, a robot wrist and a turnable robot body, employing electric drive motors (electric servomotors) as drive sources are in use. To enable such an industrial robot to perform robot motions, a robot control unit inputs commands or instructions to respective servomechanisms to electrically drive electric drive motors and mechanically drive the robot functional units according to a control program previously taught to and stored in the robot control unit, and the servomechanisms control the electric drive motors to thereby drive the functional robot units for movement about corresponding articulatory axes. In such an industrial robot employing electric motors as drive sources for moving the robot units about the corresponding articulatory axes, the motion ranges of the robot functional units about the articulatory axes, the operation speeds of the robot units, and the allowable loads on the robot units are limited by the electric rated performance of the corresponding electric drive motors Namely, an accident, such as abnormal overheating and resultant burning of the electric drive motor associated with an articulatory axis when an electric current exceeding the rated value is supplied continuously to the electric drive motor, to meet a required power exceeding the rated power thereof, may occur when carrying out a series of robot motions according to the control program to shift the working point thereof from a given position in the operating space to a subsequent desired position.
Nevertheless, the conventional industrial robot depends on a very primitive means of preventing such a possible accident. That is, an alarm is given when an excessive electric drive-current is supplied to the electric motor and the electric motor becomes overheated during a robot motion due to an excessive load on the industrial robot, or when an inappropriate set motion range or an inappropriate set operating speed occurs, and the conventional preventive means carries out a cumbersome procedure to determine the cause of the alarm through a trial-and-error method including measuring the electric drive current supplied to the electric drive motor, by a current measuring device, i.e., an ammeter, while the electric motor is in operation, recording the output of the current measuring device by a recorder, calculating the level of the actual electric drive current supplied to the electric drive motor with respect to the rated current of the electric drive motor, by a suitable calculator, and deciding whether or not the set operating speed and set acceleration of the related robot unit about the articulatory axis are appropriate. Accordingly, it has been impossible to quickly ascertain the propriety of the instant motions of the industrial robot at an optional moment in the operation of the industrial robot, and to change the control program when necessary. Furthermore, heat generated by the electric drive motors may cause a thermal distortion of a robot functional unit, such as a robot arm, which is a serious problem when the respective robot functional units must move with a high accuracy.